1. Field of the Invention
The present invention relates to field effect devices and circuits and more particularly, to devices and circuits intended to control current flow and block voltages in two opposing polarities.
2. Prior Art
A discrete field effect transistor comprises source and drain regions spaced apart by an intervening base region and includes an insulated gate electrode disposed adjacent the base region between the source and drain regions to control the conductivity of the channel region extending between the source and drain regions. Such a discrete field effect transistor is a three terminal device having source, drain and gate electrodes with the source electrode in ohmic contact with both the source region and a portion of the base region which is remote from the channel region. Thus, the source electrode short circuits the portion of the source/base PN junction it extends across. As a consequence, field effect transistors of this structure can block high voltages in only one direction since only their base/drain PN junction is capable of supporting reverse bias voltages.
There is a need for AC switches capable of controlling the flow of AC current in either direction in relatively low voltage applications with low ON-resistance. That is, applications in which the voltage applied across the field effect transistor in the OFF state is less than about 25-50 volts and the voltage drop across the transistor in the ON state is small, typically less than about 10 millivolts.
Accordingly, it is a primary object of the present invention to provide a field effect transistor AC switch which has a low ON-resistance and which is capable of blocking both polarities of an alternating voltage.
It is another object to provide an AC switch in which a single control signal can control conduction independent of the polarity of the signal applied to main terminals of the device.
It is a further object to provide a field effect transistor AC switch employing only a single field effect device.
It is a still further object to provide a field effect transistor circuit capable of controlling current flow in both polarities while exhibiting a minimum ON-resistance.